Acrylic polymers are widely used as the basis for adhesive materials. They are manufactured and applied as solvent-based, water-based or hot melt-based adhesive materials. To maintain low viscosities for ease of application, lower molecular weight acrylic polymers or acrylic monomers are commonly supplied as an uncured adhesive material and cross-linked after being applied thereby increasing the molecular weight and cohesive strength of the final adhesive composition. These post treatments are used to control the final adhesive performance.
Most cross-linkable acrylic adhesives are used without other additives, but in some cases, tackifier resins are used to improve adhesive performance. Particularly, there is one form of cross-linking mechanism used, high-energy radiation in the Ultra Violet (UV) and Electron Beam (EB) wavelength regions, where the use of tackifier resins is beneficial to improve adhesion. Commercially available tackifiers require specific properties, e.g. polarity or aromaticity, to be suitable for modification of acrylic polymers. Some examples of these commercially available tackifiers are rosins, rosin esters, hydrogenated rosin esters, pure monomer resins, styrene-based resins, and hydrogenated hydrocarbon resins. The commercial range of suitable tackifier resins is however limited. This absence of suitable tackifier resins that enable optimized adhesive performance as measured by tack, peel and cohesion have severely hampered the commercial growth of UV hot melt acrylic adhesive systems.
For example, the adhesive industry has a perception that rosin-based materials can cause skin sensitization. In particular, for the medical pressure sensitive adhesive markets, there is an unmet need for tackifier resins that are not skin sensitizers, have no cytotoxicity, and are hypoallergenic.
More specifically, new tackifier resins are also needed to improve the performance of UV curable hot-melt acrylic copolymers under the trade name, acResin®, produced by BASF. AcResin® acrylic copolymer is an environmentally friendly, solvent-free alternative to solvent-based acrylic pressure sensitive adhesives. Two of the three commercially available acResin® acrylic copolymers were developed to be formulated with suitable tackifier resins to obtain the correct adhesive properties for various pressure-sensitive adhesive end-uses. Currently, the acResin® UV technology platform lacks degree of freedom to enable the many adhesive formulators to design tailor-made solutions for specific end uses. With 3 acResin® UV curable, hold-metal acrylic copolymers and a limited choice of suitable, commercially available tackifier resins, there is a need in the adhesive industry to design novel tackifier resin types for optimum compatibility with acResin® UV technology.
Therefore, there is a commercial need for compatible tackifier resins for radiation-curable compositions in the adhesive industry to produce radiation-curable adhesives.